1. Field of the Invention
The present invention relates to a thermal printing method and thermal printer. More particularly, the present invention relates to a thermal printing method and thermal printer in which irregularities in a recorded density are prevented from occurrence.
2. Description Related to the Prior Art
A thermal printer includes a thermal head, which applies heat to recording material to print an image. There are two examples of thermal printers, including a direct thermal recording type in which the thermal head heats thermosensitive recording paper to color it directly, and a thermal transfer type in which a back surface of ink ribbon is heated by the thermal head to transfer ink to paper.
To record a color image according to the direct thermal recording, a thermosensitive recording paper called "Thermo-Autochrome paper" is used. The recording paper includes a support and at least three thermosensitive coloring layers. The coloring layers are cyan, magenta and yellow coloring layers. The recording paper and the thermal head are conveyed relative to one another in a sub scan direction. The thermal head presses and heats the recording paper to record a full-color image. To color the coloring layers selectively, the recording paper has heat sensitivity different between the coloring layers. The cyan coloring layer is positioned the most deeply, and has the highest heat sensitivity. The yellow coloring layer is positioned the least deeply, and has the lowest heat sensitivity. Before recording to the next one of the coloring layers, a previously colored one of the coloring layers is fixed by application of ultraviolet rays, and prevented from being colored for higher coloring density.
The thermal head includes an array of numerous heating elements arranged in a main scan direction, for recording one line after another of each of the colors. For this operation, the heating elements apply bias heat energy to the recording paper. The bias heat energy is such an amount that it heats the recording paper to set the recording paper in a state short of developing color and prepares it for further application of heat energy. Then the heating elements apply image heat energy to the recording paper. The image heat energy is an amount for coloring at a desired density. Pixels arranged on the recording paper virtually are colored to record dots. The bias heat energy is constant and depends on each of the coloring layers. The image heat energy changes and depends on input image data representing a gradation data. When the image heat energy is finished, the heating elements are left to stand in a cooling period. After the heating elements are cooled, one other line is recorded.
A coefficient .mu. of friction between the thermal head and the recording paper changes and depends upon a surface temperature of the recording paper. If the temperature of the vicinity of the heating elements in the thermal head is low, the coefficient .mu. of friction is great. Load to the conveyance is high. If the temperature of the vicinity of the heating elements is high, the coefficient of friction is small. The load to the conveyance is low.
When the temperature of the thermal head is changed to increase or decrease the load to the conveyance, there is an increase or decrease in distortion of the platen roller or platen drum, distortion of a feed roller or conveyor roller set, extension or shrinkage of belts for transmission of rotation to the conveyor roller set, and distortion of a roller shaft. A paper conveyor system for the recording paper is associated with a stepping motor as a power source. A rotor of the motor may be stopped in a position different from an accurate stop position, but where the magnetic force and the load to the conveyance are balanced. Note that such recoverable changes in the state are herein referred to as distortions of the paper conveyor system. At each time that the load to the conveyance changes, the distortions of the paper conveyor system are either increased or decreased. Then a conveying speed of the recording paper changes in a temporary manner.
If the conveying speed is higher than a predetermined, value an interval between recorded lines is increased. Heat energy (mJ/mm.sup.2) applied by the heating elements to the recording paper per unit area is small. Coloring density of a line being recorded becomes low, to create a low-density stripe which looks blank or white. As the conveying speed is lower than a predetermined, value the interval between adjacent lines is lowered. The coloring density of the line being recorded rises, to create a high-density stripe which looks dark or black. Those high- and low-density stripes constitute unevenness in the coloring density.
It is conceivable that a synthesized image is printed by combining an input image, a template image preset to be printed about the input image, and a frame to be disposed about the input image and inside the template image. In other words, the frame constitutes a periphery of an insertion region into which the input image is inserted inside the template image. The frame may have portions extending in parallel with the main scan direction. When the heating elements are positioned at portions of the frame, the heating elements have a relatively low temperature. If an image surrounded by the frame has a high density, portions for such an image are positioned at the heating elements so that a greater part of the heating elements abruptly comes to generate high heat energy. The temperature of the thermal head rises abruptly. The surface temperature of the recording paper in contact with the heating elements rises. The friction coefficient decreases. The conveying speed of the recording paper becomes higher temporarily than a predetermined value. There occurs a low-density stripe where the coloring density of a line is considerably low.
If blank portions of the frame are printed immediately after printing a high-density portion, the conveying speed of the recording paper becomes lower temporarily than a predetermined value. A high-density stripe occurs, in which the coloring density of a line being recorded is remarkably high.